Femoral length is an obstetric ultrasound measurement of the length of the fetal thigh bone. A low femur length usually indicates a risk of intrauterine growth retardation or Down’s syndrome. However, the length of the femur should not be used alone to determine the size of the fetus, but also the head circumference, biparietal diameter and abdominal circumference. Therefore, it is recommended to improve other data, and then the doctor will determine whether the condition is normal or not, and if there is an abnormality, timely intervention is needed.